collectors/charts.d/opensips: fix detection of `opensipsctl` executable (#10978)

collectors/charts.d.plugin/opensips/opensips.chart.sh

@@ -31,6 +31,7 @@ opensips_check() {
   # try to find it in the system
   if [ -z "$opensips_cmd" ]; then
     require_cmd opensipsctl || return 1
+    opensips_cmd="$OPENSIPSCTL_CMD"
   fi
 
   # check once if the command works